1. Field of the Invention
The present invention relates to a method of exposure control and an image pickup device realizing the exposure control method.
2. Description of the Related Art
Image pickup apparatus such as digital cameras comprise a solid-state image sensor such as a charge coupled device (CCD) or a complementary metal oxide semiconductor (CMOS). The solid-state image sensor is provided in a lattice-like arrangement on an imaging plane. The image sensor is exposed to light and electrical charge by light reception is sequentially transferred to be read out by a signal control section or the like so that an image is taken. Subsequently, an analog signal is converted into a digital output value by AD conversion. A white balance correction and the like are applied to the image signal, whereupon a static image (hereinafter, “frame”) to be delivered to a display device etc. is formed. A time for exposure of an image sensor to light is controlled by a shutter speed of an electronic shutter in the image pickup apparatus. A charge transfer interval or charge readout timing is controlled by the signal control section or the like so that an exposure value of image to be taken is controlled, whereby the luminance of output image is controlled.
The aforenoted frame is formed at a predetermined number of times per second in digital cameras and the like (hereinafter, “frame rate”), thus composing an image. A frame rate at the image output side is fixed to a predetermined number of times in a range from 30 to 60 times per second. At the input side of the signal control section, a charge transfer interval or charge readout timing is controlled, whereby a frame rate is controlled so as to be varied, whereby the same effect as achieved by a diaphragm mechanism (mechanical iris).
Furthermore, an exposure value depends upon an amount of light of an object and around the object (hereinafter, “object luminance”) obtained by a photometric function of the image pickup apparatus. When the object luminance has been changed, the shutter speed of the image sensor or an input side frame rate is controlled to be switched to a previously set value by an automatic exposure control (hereinafter, “AE control”).
The AE control is generally carried out after a focal position has been determined by an automatic focus control (hereinafter, “AF control”) unit in which a focus lens is moved to a preset position with movement of a zoom lens. Furthermore, AF control manners include a mountain-climbing type in which a zooming speed is controlled by a zoom switch or the like so that a focus lens is moved as soon as possible relative to the position of the zoom lens, thereby being focused on the object. An exposure value set according to the position of the focus lens is supplied to a signal control section or the like when a zooming speed is controlled by the AF control or the like. As a result, there arises no problem resulting in excessive or insufficient exposure.
However, the following two problems arise when the zoom lens is manually operated at high speeds. In the first problem, when the zoom lens is moved from a wide-angle end to a telephoto end, the AE control cannot follow the zooming speed, whereupon exposure becomes excessive such that an output image becomes white or flies white. Furthermore, in the second problem, when the zoom lens is moved from the telephoto end to the wide-angle end, the AE control cannot follow the zooming speed alike, whereupon exposure becomes insufficient such that an output image becomes dark.
In view of the problems, JP-A-H11-183778 discloses a control method of comparing zooming speed information with a threshold value. When a zoom lens is moving at a speed exceeding the threshold value, it is determined that an amount of peripheral light is rapidly changing. In this case, a maximum aperture diameter of a diaphragm is changed in accordance with a focal distance.
However, in the case where the maximum aperture diameter of the diaphragm which is based on the position of the zoom lens is controlled in accordance with the zooming speed, photometry and AE control need to be re-executed upon completion of zooming when the luminance on an imaging plane of the image sensor (hereinafter, “focal luminance”) does not agree with the luminance corresponding to a previously set maximum aperture diameter of the diaphragm. In this case, there is a possibility that the brightness may change as described above although the change is transient, whereupon the changes in the focal luminance cannot sufficiently be suppressed.